Changing a funding account is only possible when there are no pending transfers between your external funding account and your Qapital account, regardless of the direction of the transfer.
Pending transfers usually take 1-2 business days to clear. If you want to make sure you won't get additional pending transfers, you can always pause your rules by going to the Transfers tab > Rules, and toggle the Snooze button. Once your Rules are paused and all your pending transactions settled, you can try and change your funding account again.